Hi, Although I am not exclusively a Surfboard rider - I'll happily ride my 5'8" directional on a 12M and will also be considering my 16 if there's a good enough swell with constant (albeit light) winds this summer.

I don't see any reason to restrain on the low end - as long as the kite isn't falling out of the sky, & there's a nice swell, why not fly?

14 is my biggest kite and a board with a bit of volume works well.
If you have proper groundswell then the waves will be perfect in a sideshore light wind, Anything less than a ten and the waves start to get a bit choppy unless you are lucky enough to have a reef or sand bar to keep the chop to a minimum...

Although my most used kite is a 7 and either it or the 5 is my favourite I also have 12 and 16 metre kites.
I usually use the 16 with a TT but have had some great sessions with it on a surfboard in small waves, I love being the only one out in a clean lineup.
I find the Switch Nitro 16 turns reasonably fast for a big kite and if you have a good cross shore you can just unhook and let it drift slowly down the line.

A 17m Fat Lady with a 5'11" Fish. Mostly upwind wave riding though which is still a blast if there is any kind of swell. Can get a little sketchy if the wind gets too light/onshore and there is size. Mainly because it's easier to outrun your kite. Anything over 11 mph is doable as long as your kite drifts well and not prone to falling.

Depends on where you are. In the Great Lakes there is no "Ground swell" , whatever waves there are are a result of the wind you are experiencing at the moment. So no light wind wave riding. That said the biggest I have used lately is a 10 but for the most part the waves aren't worth getting excited about until it is 8/9meter wind.